I think they figured out the ballooner must have swallowed some evidence. So it'd be in his stomach or show up on the autopsy.
Hmmm. That seems highly-likely; TYVM! Corpsetaker was a mind-magic specialist, could easily have learned that whatsizname
was a ballooner, and that his corpse would have the info.
I bet they could pull a much-more refined & informative version of Molly's "look into the eyes of the dead" trick, too.